Various processes and equipment for extrusion foaming of thermoplastic resins have been used for many years. Generally, solid pellets of thermoplastic resin are fed through a hopper to a melting zone in which the resin is melted, or plasticized, to form a flowable thermoplastic mass. The plasticized thermoplastic mass generally is then transported to a mixing zone where the thermoplastic mass is thoroughly mixed with a blowing agent under pressure for subsequent cooling and substantially free expansion of the resin to form a foam. The blowing agent expands the molten mass to form the cells of the foam and the thermoplastic foam is cooled.
The blowing agent gradually diffuses from the cells of the foam and is eventually replaced by air diffusing into the cells. Diffusivity of the blowing agent and the selection and use of appropriate blowing agents are important aspects of foam manufacture. If the diffusivity of a blowing agent out of the cells of a foam is too fast compared to the diffusivity of air, so that the blowing agent is not replaced by air as it escapes, then the foam typically collapses and is said to have dimensional stability problems. For many years, chlorofluorocarbons ("CFCs") were used that had excellent diffusivity characteristics and resulted in high quality, dimensionally stable foam products. However, CFCs are no longer acceptable as blowing agents because of global regulations prohibiting their use.
Other compounds, including lower hydrocarbons, alcohols and ketones, various hydrofluorocarbons, and inert gases have been proposed as alternative blowing agents to CFCs. Some of these compounds diffuse out of the foam cells at a rate that reduces dimensional stability and can result in collapse of the foam cells. Aging modifiers have been developed for incorporation into polyolefin resins that slow the diffusion of selected blowing agents out of the polyolefin foam cells. These aging modifiers are sometimes referred to as permeability modifiers or stability control agents. Among the various aging modifiers that have been proposed are the saturated higher fatty acid amides, saturated higher aliphatic amines, esters of saturated higher fatty acids, copolymers of ethylene and unsaturated carboxylic acids, and others.
One of the more widely used aging modifiers for polyolefin foams is glycerol monostearate. Glycerol monostearate is also added to resins for extrusion to form films. Glycerol monostearate is also called GMS, glyceryl monostearate, and monostearin. GMS is a monoglyceride and is an ester of stearic acid, which is C.sub.18 acid, and the trihydric alcohol glycerol. GMS is a pure white or cream colored and wax-like solid.
GMS and other similar aging modifiers are thought to coat the walls of the foam cells to slow the blowing agent gas from escaping and thereby to prevent collapse of the cells of the foam. However, these aging modifiers can leave a grease-like residue on the surface of the foam that can be transferred to objects that come into contact with the foam. The transfer of this grease-like residue to certain substrates is problematic and is particularly undesirable on optical products and high gloss finishes.
GMS also tends to slow the rate of blowing agent diffusion from the cells to the point that some residual blowing agent is maintained in the cells for an undesirably long period of time after manufacture. Too slow a rate of blowing agent diffusion from the cells of the foam can result in dimensional stability problems. Foam rolls can become tight in storage.
It would be desirable to provide a foam product in which an acceptable blowing agent would escape from the cells of the fresh foam at a rate that more closely matches the rate of air entering the cells of the foam to substantially eliminate dimensional changes in the foam upon aging. It would also be desirable to provide foam and other products, including films, in which transfer of grease-like residue to substrates is reduced.